Fluororubbers exhibit excellent chemical resistance, solvent resistance and heat resistance, among other properties, and, as such, find application in a broad range of uses, for example as sealants which are fully serviceable under rugged conditions.
Fluororubbers usually comprise polymers having pre-cure average molecular weights within the range of about 50,000 to 200,000 and compositions comprising such polymers together with a crosslinking agent and other formulating agents are so high in viscosity that the manufacture of shaped articles requires fluidizing the compositions at elevated temperatures in the molding-curing stage.
However, with such high-viscosity compositions, it is difficult to obtain shaped articles having complicated geometries. A further disadvantage is that because of the impossibility of field molding, e.g. in situ curing at rubber installation sites in an automotive assembly line, a premolding procedure is essential so that the process cannot be simplified and the mode of utilization is limited.
To overcome these disadvantages, a demand exists for the development of a liquid rubber which shows pre-cure fluidity at room temperature. Such liquid rubber enables on-site molding and, moreover, can be poured into molds in a liquid state at room temperature so that shaped articles of complicated geometry can be easily obtained.
As such liquid rubbers, silicone rubbers as heat-resistant rubbers of the on-site application type are commercially available. However, from oil and solvent resistance points of view, the advent of fluorine-containing liquid elastomers has been awaited in earnest. Silicone rubbers present with many problems arising from the liberation of siloxanes in uses requiring cleanliness but the release of siloxane gases cannot be prevented because of the degradation inherent in the molecular structure. Even if the rubbers are purified in the production stage, the reduction in the amount of liberation of siloxanes is limited.
Fluororubbers comprising polymers having high molecular weights may have sufficient elastomeric properties prior to crosslinking because the crosslink interval of the trunk chain can be increased and, even after curing, the trunk chain between the crosslinks has flexibility. In contrast, liquid fluororubbers comprises polymers having low molecular weights providing for fluidity at room temperature prior to crosslinking and usually have short crosslink intervals, with the consequence that the post-cure elastomeric properties are inadequate.
When the fluororubber comprising a polymer having a high molecular weight prior to crosslinking does not have crosslinking sites at both ends of a trunk chain but has such sites in intermediate positions, the trunk chain segment from the crosslinking site to the chain end is not directly involved in the formation of a three-dimensional network but has a certain degree of freedom. However, since it is inherently a high polymer, the presence of this trunk chain segment has no material influence on the cured article.
In contrast, when the crosslinking site of a liquid fluororubber is not situated at the trunk chain end but in an intermediate location, the trunk chain segment from the crosslinking site to the chain end acts as if it were a plasticizer because of the low molecular weight of the liquid rubber, with the result that the mechanical strength of the cured article is decreased.
In order to overcome these disadvantages and carry out the curing reaction with good efficiency and improve the moldability of liquid rubber, it is desirable to ensure that the trunk chain of the polymer constituting the liquid rubber has cure sites substantially at both ends of a trunk chain. As regards the technology for introducing a functional group serving as said cure site into the trunk chain end, several methods have been proposed as follows.
Japanese Kokoku Publication Sho-63-44744 discloses a method which comprises introducing a t-butoxy group into the trunk chain end of a vinylidene fluoride [VdF]-hexafluoropropylene [HFP] copolymer by means of a certain fluorine-containing diacyl peroxide and converting the same group to a hydroxyl group with an acid. However, this polymerization initiator is a special and expensive reagent and, moreover, must be used in a large amount for production of a low-molecular-weight polymer, thus being disadvantageous cost-wise.
As an analogous technology, the method comprising using a fluorine based peroxide is disclosed in U.S. Pat. No. 3,291,761 but this method is also costly.
As a technology for producing a fluororubber having a cure site at both ends of a trunk chain, Japanese Kokai Publication Hei-08-67660 and Japanese Kokai Publication Hei-11-286541 disclose processes involving the use of a perfluorodicarboxylic acid fluoride as a polymerization initiator. However, this technology is also disadvantageous cost-wise because of the high price of the polymerization initiator.
Japanese Kokai Publication Sho-56-57811 discloses a method of introducing an iodine atom into the trunk chain end of a liquid fluorocopolymer comprising a VdF copolymer which comprises using an iodine compound as a chain transfer agent. This technology is seriously handicapped in production cost, for it requires a large amount of the expensive iodine compound and, moreover, because the highly reactive iodine is contained in a proportion of several mass %, the transformation to a different functional group by a polymer reaction is required.
Japanese Kokai Publication Hei-11-322842 discloses a method for producing a fluorinated oligomer having a carboxyl group at both ends of a trunk chain which comprises causing a crosslinked fluororubber to swell with an organic solvent and decomposing it in the presence of a base and a peroxide. However, there is the problem that while this decomposition is feasible for a crosslinked article comprising VdF polymer, it cannot be applied to perhalogenated elastomers.
As a technology for introducing a hydroxyl group into the trunk chain end, Progress in Polymer Science, 26, 2001, p. 105-187 describes a method which comprises preparing a VdF/HFP copolymer having an average molecular weight of about 4000 in the presence of hydrogen peroxide and causing LiAlH4 to act upon the copolymer. However, as the hydroxyl radical reacts with the CF2 side of CF2═CH2 in the above copolymerization, the unstability of the hydroxyl group causes the reaction to proceed further so that actually the hydroxyl radical is hardly introduced into the chain end.
Generally the technology for introducing a functional group into the trunk chain end of a polymer makes it necessary to adopt such special techniques as above and is, therefore, costly and limited in the scope of application, so that it is deficient in practical utility.
The pamphlet of WO 00/29479 discloses a fluorine-containing elastomer having a carboxyl group at the trunk chain end. This elastomer is, for instance, a substance having a large molecular weight as can be seen if only from the infrared absorption spectrum in FIG. 1 of the pamphlet which indicates a relatively low carboxyl group concentration compared with the carbon-fluorine bond concentration and, furthermore, it is an elastomer, that is to say a high polymer having rubbery elasticity at room temperature, and, therefore, has no fluidity at room temperature.